CHICAGO (Reuters) - President Barack Obama will announce on Monday that the Department of Agriculture intends to buy up to $170 million of pork, lamb, chicken and catfish to help support farmers suffering from the drought, a White House official said.

The food purchases will go toward "food nutrition assistance" programs, like food banks.

During a visit to Iowa, a political swing state that the Democrat hopes to win in the November 6 election, Obama will press Congress to pass a farm bill with short-term relief measures for the ranchers and farmers hurt by the drought.

The president will also direct the Department of Defense to "encourage" its vendors to speed up purchases of lamb, pork and beef and freeze it for later use.

"This is a win-win. Farmers and ranchers will have an opportunity to sell more of their products at this critical time and taxpayers will get a better price on food that would have been purchased later," the official said.

"The president has directed his administration to continue exploring every possible avenue to provide relief to communities struggling with this historic natural disaster."

Obama will tour an Iowa farm to view the effects of the drought. The state is one of a handful of political swing states, including Ohio, Florida, and Colorado, that could hold the key to victory in his race with Republican Mitt Romney for the White House.

Last week the governors of two poultry-growing states, Maryland and Delaware, asked the Obama administration for relief from the requirement to use corn ethanol in gasoline, saying corn is needed to feed livestock.

(Reporting by Jeff Mason; Editing by Eric Beech and Stacey Joyce)